Question using an AES decryption

Greetings, 

first off this is library is awesome stuff! 

I would like to encrypt some text with AES. Following the DES example in 
the Crypto manual I have the following piece of code:

--------------
# Encrypt some text with 256bit AES
from Crypto.Cipher import *
plain="this is some textXXXXXXXXXXXXXXX"  # make text len multiple of 16
obj = AES.new('1234567812345678', AES.MODE_CBC, 'bbbbbbbbaaaaaaaa')
print "Before encryption: ", plain
ciph = obj.encrypt(plain)
print "After decryption: ", ciph
print "After decryption: ", obj.decrypt(ciph)
--------------

Here's the result I'm getting:

--------------
Before encryption:  this is some textXXXXXXXXXXXXXXX
After encryption:  ¸9Œ†¾{.'PÃWÄ	t”æeäVñ¤Bx•¹QÚ–
After decryption:  ‚ìnõúµYPžt‘øDÞtXXXXXXXXXXXXXXX
--------------

The decryption doesn't work as expected, I'm not receiving my original 
text. I guess I'm passing the wrong parameters to instantiate my AES 
ojbject...

Any ideas?

Thanks in advance for your help.

Alex